Water-containing formulations and substrates, for example, paints and coating formulations, latex emulsions, inks, adhesives, sealants, joint compounds and concrete, are susceptible to microbial attack when exposed to common environmental conditions. Organisms causing spoilage or defacement include bacteria, yeast, fungi and algae. These microorganisms may degrade or impair different properties and attributes of such formulations and substrates by affecting their pH, viscosity, color, odor and rheology, among others.
The use of biocides to protect these formulations and substrates is of paramount importance. There are continuous efforts being made to develop more effective, less toxic and more economical biocides. Synergistic mixtures provide many of these desirable attributes.
Synergistic antimicrobial activity exists when the combination of two or more antimicrobial compounds results in the use of lesser amounts of each, to bring about the same inhibitory effect than the use of either compound acting alone. The synergistic interaction thus produces an effect that is more than additive in the resultant antimicrobial activity.
The compound 2-propenal polymer or copolymer is disclosed in U.S. Pat. No. 6,060,571 as being useful in aqueous systems as a biocide and is available commercially as NUOSEPT APC from Degussa. Its antimicrobial activity is linked to the release of free monomeric 2-propenal. 2-propenal polymer (APC) is a broad-spectrum biocide used to protect various products including aqueous dispersions, emulsion paint coatings, polymers for latices, amine-free cutting oils and cooling water circuits. It is an effective biocide against various bacteria, yeast and algae.
Isothiazolones such as the mixture of 5-chloro-2-methyl-4-isothiazoline-3-one and 2-methyl-4-isothiazoline-3-one (CIT/MIT) is also commercially available (Kathon LX 1.5, from Rohm & Haas). Isothiazolones are disclosed in U.S. Pat. Nos. 3,761,488; 4,105,431; 4,252,694; 4,265,899; 4,279,762 and elsewhere. Mixtures of CIT/MIT have been used to preserve personal care, household and industrial products and show good microbiocidal activity.
The use of antimicrobial compositions which evidence synergistic activities present several advantages: an increase in effectiveness, a broadened antimicrobial spectrum of activity, a reduction of the use levels and a decrease in toxicity of a given agent to the host and the environment. The synergistic activity may be the result of each compound having a different mechanism of action on the target microorganism.
Biocidal combinations, some of which show synergistic activities have been described. Isothiazolones have been combined with: (a) 1-methyl-3,5,7-triaza-1-azonia-tricyclo(3.3.2.2)-decane chloride in U.S. Pat. No. 5,294,614; (b) a heavy metal complex (disodium monocopper (II) cifrate) in U.S. Pat. No. 4,608,183; (c) hydroxymethylaminoacetic acids, its salts and lower alkyl esters in U.S. Pat. No. 4,980,176; (d) 1,2-benzisothiazolin-3-one in EP No. WO 99/08530; (d) one or more components selected from the group consisting of p-chloro-m-xylenol, sodium dicchlorophene, bis-(2-hydroxy-5-chlorophenyl)sulfide, benzyl-bromoacetate, dodecylamine, 4-(2-nitrobutyl)morpholine and dirpopylamine in U.S. Pat. No. 5,489,588; (e) 1,3-dimethylol-5,5-dimethylhydantoin in U.S. Pat. No. 6,114,366, among others. Several of such combinations may exhibit disadvantages, either due to their cost, stability, compatibility, toxicity or other problems.
Isothiazolones also decompose easily and lose their antimicrobial activity, for example, when placed in water or in the presence of other reactive molecules. Dilute solutions of 3-isothiazolones can be stabilized with cationic salts, such as magnesium nitrate or copper nitrate. The use of nitrates as stabilizers is undesirable, however, because of their potential to react with amines and form nitrosamines, which are suspected to be carcinogens.
The stability of 3-isothiazolones can also be improved by the addition of formaldehyde or formaldehyde-donors (U.S. Pat. Nos. 4,129,448, 4,165,318 and 6,121,302). However, these types of stabilizers may also pose a problem. In some animals, formaldehyde may act as a carcinogen.
Other methods have been described to stabilize 3-isothiazolone solutions including U.S. Pat. Nos. 5,461,150 and 5,153,213, which provide methods for stabilizing solutions of 3-isothiazolone by the use of low levels of copper ion in the form of a copper salt (e.g. copper sulfate) or the use of inorganic oxidants (e.g. peroxide). Copper salts may present a problem in those applications, which are sensitive to the addition of salts containing divalent ions. Copper reacts with many anionic surfactants and forms insoluble organic salts, which cause the surfactants to lose function and may result in changes to the physical properties of the finished product. Copper is also a heavy metal, which may pollute the environment.
It is an object of the present invention to provide a synergistic biocidal combination, which is more efficacious than known microbicidal compositions and is also stable in aqueous solutions, thus overcoming the various problems described in the prior art.